Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Similarly, roller cone earth-boring rotary drill bits may include cones mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit. In other words, earth-boring tools typically include a bit body to which cutting elements are attached.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compacts (often referred to as “PDCs”), one or more surfaces of which may act as cutting faces of the cutting elements. Polycrystalline diamond material is material that includes interbonded grains or crystals of diamond material. In other words, polycrystalline diamond material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain” and “crystal” are used synonymously and interchangeably herein.
Polycrystalline diamond compact cutting elements are typically formed by sintering and bonding together relatively small diamond grains under conditions of high temperature and high pressure in the presence of a catalyst (e.g., cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer (e.g., a compact or “table”) of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high temperature/high pressure (HTHP) processes. For example, a powdered catalyst material may be mixed with the diamond grains prior to sintering the grains together in an HTHP process.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains of diamond in the resulting polycrystalline diamond compact. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use, due to friction at the contact point between the cutting element and the formation.
Polycrystalline diamond compact cutting elements in which the catalyst material remains in the polycrystalline diamond compact are generally thermally stable up to a temperature of about 750° C., although internal stress within the cutting element may begin to develop at temperatures exceeding about 350° C. This internal stress is at least partially due to differences in the coefficients of thermal expansion of the diamond table and the cutting element substrate to which it is bonded. This differential in thermal expansion coefficients may result in relatively large compressive and tensile stresses at the interface between the diamond table and the substrate, and may cause the diamond table to delaminate from the substrate. At temperatures of about 750° C. and above, stresses within the diamond table itself may increase significantly due to differences in the coefficients of thermal expansion of the diamond material and the catalyst material within the diamond table. For example, cobalt may have a linear coefficient of thermal expansion significantly higher than diamond, which may cause cracks to form and propagate within the diamond table as the diamond table heats, eventually leading to deterioration of the diamond table and ineffectiveness of the cutting element.
Furthermore, at temperatures at or above about 750° C., some of the diamond crystals within the polycrystalline diamond compact may react with the catalyst material, causing the diamond crystals to undergo a chemical breakdown or back-conversion to another allotrope of carbon or another carbon-based material. For example, the diamond crystals may graphitize at the diamond-crystal boundaries, which may substantially weaken the diamond table. At extremely high temperatures, in addition to graphite, some of the diamond crystals may be converted to carbon monoxide and/or carbon dioxide.
In order to reduce the problems associated with different coefficients of thermal expansion and chemical breakdown of the diamond crystals in polycrystalline diamond compact cutting elements, so-called “thermally stable” polycrystalline diamond compacts (which are also known as thermally stable products, or “TSPs”) have been developed. Such a thermally stable polycrystalline diamond compact may be formed by leaching the catalyst material (e.g., cobalt) out from interstitial spaces between the interbonded diamond crystals in the diamond table using, for example, an acid or combination of acids (e.g., aqua regia). All of the catalyst material may be removed from the diamond table, or catalyst material may be removed from only a portion thereof. Thermally stable polycrystalline diamond compacts in which substantially all catalyst material has been leached out from the diamond table have been reported to be thermally stable up to temperatures of about 1,200° C. It has also been reported, however, that such fully leached diamond tables are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are non-leached diamond tables. In addition, it is difficult to secure a fully leached diamond table to a supporting substrate.